Antimicrobial compositions are generally added to various kinds of aqueous fluid media to reduce or inhibit the growth of microorganisms. In particular, a wide variety of industrial aqueous fluid media such as metalworking fluids used with metalworking equipment require antimicrobial compositions to control the growth microorganisms which eventually render the fluids rancid.
The development of high speed metal cutting and grinding has resulted in the creation of lubricants containing oils and chemicals stabilized in water. These fluids impart the cooling qualities of water and the lubricating properties of oil which prolong the life of cutting tools, reduce heat production, improve surface finish of the metal being machined, prevent warping and leave a rust-inhibiting film of oil on the worked piece.
Normally these fluids include fatty or petroleum oils, soaps, or synthetic based materials and additional additives such as antifoam agents, preservatives, coupling agents and rust inhibitors. The coolants are generally marketed in the form of concentrates which are normally diluted with water by the user in ratios of 1 part oil to about 20-40 parts of water, but these ratios may vary with particular operations. The lubricant is supplied to a machine from either an individual tank containing 50 to 100 gallons or from a large sump containing thousands of gallons which supplies many machines.
One of the problems often associated with such aqueous fluid media arises from the susceptibility of the media to the infestation and growth of various microorganisms such as bacteria and fungi (which particularly feed on the organic components thereof). The presence and buildup of such microorganisms can often lead to interference in the metalworking operations as a result of the clogging of filters, buildup of slime and sludge, development of odors, rust, emulsion instability, reduced tool life and poor finish. Furthermore, in machine shops where the workers' hands necessarily come in contact with the cutting oil, usually containing finely divided sharp metal cuttings, serious problems of dermatitis may arise. These and other such similar problems have resulted in the continuing need for better antimicrobial additives for aqueous fluid media such as metalworking fluids. Much effort has been devoted in recent years to controlling this problem; however, it continues to be a major annoyance which costs the metalworking industry many millions of dollars each year.
A number of suggestions have been made for inhibiting the growth of bacteria in aqueous fluids such as those described in U.S. Pat. Nos. 4,172,140, 3,951,830, 3,799,876, 3,515,671, and 2,976,244. The use of various formaldehyde preservatives for metalworking fluids including monomethylol dimethyl hydantoin and dimethylol dimethyl hydantoin has also been proposed (see Bennett, E. O., Int. Biodetn. Bull. 9: 95-100 (1973) and Maeda et al, Agr. Biol. Chem., 40: 1111-2222 (1976)).
Gray and Wilkinson in J. Gen. Microbiol., 39: 385-399 (1965) and J. App. Bact., 28: 153-164 (1965) describe the action of the ethylenediaminetetraacetic acid (EDTA) on some bacteria. The effectiveness of such chelating agents as EDTA along for bacterial control in aqueous systems is disputed as evidenced by U.S. Pat. Nos. 3,240,701, 3,408,843, and 3,591,679.
The antimicrobial compositions used in metalworking fluids seem to suffer from one or more disadvantages including high cost, unacceptable toxicity or low degree of effectiveness at the present state of the art.
Considerable advantages would result if a coolant could be formulated with an ingredient which has several different functions in regard to metalworking while, at the same time, exhibiting antimicrobial properties to provide partial or complete rancidity control. In this way, it might be possible to partially control increasing costs of these lubricants, as well as provide increased life under industrial conditions.
Antimicrobial agents and corrosion inhibitors constitute two important ingredients of metalworking fluids which commonly are depleted faster than the other components of the products. Quite often, both of these materials must be added to a coolant at periodic intervals in order to compensate for their loss from the coolants.
Preservatives are removed from the fluid as the chemicals combine with the microbes to bring about their inhibition or death. The greater the microbial population, the more cuickly they are lost from the system. Thus, the concentration of any preservative declines with time and may be reduced to subinhibitory levels in only a few weeks.
Rust inhibitors have an ability to absorb to metal surfaces. They usually coat the metal being worked, as well as the surfaces of the machine and circulation system. They sometimes even prevent the coating of metals with the oils commonly encountered in cutting fluids. Thus, the concentration of the rust inhibitor in a cutting fluid also declines with time as it is removed from the system on the metal parts being worked.
It would be worthwhile then to search for chemicals which can function both as corrosion inhibitors, as well as antimicrobial agents.